The Manhattan Project was a 6-year effort to develop a nuclear fission weapon before the Nazis could, sparked by a warning from European scientists to US President Roosevelt.

The Manhattan Project had its roots in a single letter sent from a group of Hungarian scientists to President Franklin Delano Roosevelt with the help of world-renowned physicist Albert Einstein. This letter outlines what they saw as a very real and very pressing problem confronting the U.S. and the world -- that Hitler and Germany might develop a weapon based on nuclear fission. This letter led to the development of a small research team to begin a United States effort to build the bomb first ... [1]

The term Manhattan Project was the code word for the project that developed the first nuclear weapons - "atomic bombs" as they were then called - in history. The formal code name was "Manhattan Engineering District," named after the Manhattan Engineer District of the United States Army Corps of Engineers. Manhattan borough had at least 10 sites that supported the project, including the project's first headquarters located in a skyscraper across from City Hall.[1]

It had been known since about 1900 that the natural decay of radioactive elements produced quantities of energy that were enormous compared to those produced by chemical reactions. The problem was that natural processes liberated it very slowly; for example, the half-life of radium is 1602 years, so a lump of radium will stay warm for thousands of years, but is not warm enough to be a useful source of energy. In 1914, H.G. Wells wrote a science-fiction novel, The World Set Free, describing "atomic bombs," in which the release of energy had been artificially speeded up; his description of the process was, however, imaginary gobbledegook. In 1933, Hungarian-born physicist Leo Szilard invented the idea of the nuclear chain reaction which made the rapid release of atomic energy a theoretical possibility. The idea was in the air, and in fact during the Second World War, Japan, Germany, USSR, and the United States all worked on atomic bomb projects, although the Japanese and German projects did not get far.

Leo Szilard, who had immigrated to the United States, was determined to persuade President Franklin Roosevelt to fund the building of an atomic bomb.[2] He felt that a letter to the President by Albert Einstein would help. Einstein, a pacifist, was known better than most physicists. Einstein agreed to sign a letter written by Szilard, which was then taken to President Roosevelt in early August 1939. The letter explained the feasibility and importance of developing an atomic bomb.

But President Roosevelt did little based on this letter, and instead only allocated a tiny $6,000 for such a project. Over two years passed without progress. It was not until Dec. 6, 1941, the day before Pearl Harbor, that an all-out effort to build an atomic bomb began under the direction of General Leslie Groves. This was what became known as the Manhattan Project.

Groves was bold and willing to spend money. When scientists and engineers were unable to decide which of three approaches to producing the fissile material needed to build the bomb, he decided that if nobody could tell which was best, the thing to do was try all three methods. Huge manufacturing facilities were built at Oak Ridge, Tennessee and Hanford, Washington. Assuming that fissile material would be ready, a full-scale effort to design and build the bombs themselves was begun in spring 1943 at the remote desert location of Los Alamos. The scientific and engineering efforts at Los Alamos were managed by J. Robert Oppenheimer.

The first completed bomb was tested on July 16, 1945 at a location code-named "Trinity." The other two bombs were dropped on Hiroshima and Nagasaki Japan, and ended World War II.

Despite tight security, Communist spies leaked information to the Soviet Union who soon started a similar project.

Sources:

↑Robert S. Norris, "The Manhattan Project" (Black Dog & Leventhal). The book claims that spies stole some of the project's top secrets from Manhattan.